This invention relates to the production of tomographic X-ray images and more particularly to methods and apparatus for generating data from which visible images may be derived that display a cross-sectional or oblique sectional radiographic view of a subject taken along one or more planes through the subject.
While the invention will be herein discussed primarily with reference to medical radiology for purposes of example, it should be understood that the method and apparatus are equally adaptable to the X-ray analysis of subjects other than medical patients or other living organisms. The invention is equally applicable to the inspection of metallurgical castings for internal flaws, for example, and to other usages in which the internal structure of an inanimate object is to be studied.
The conventional X-ray image of a subject as used for medical diagnostic purposes or the like has a serious disadvantage in that the visible image which is produced on a sheet of photographic film or on a fluorescent screen is not representative of a single sectional plane through the subject but instead contains overlapping or superimposed images of various organs, bones and other regions of differing X-ray transmissiveness which are distributed throughout the volume of the subject. Consequently, it is often difficult or impossible to obtain certain desired information from the image. Further, conventional radiology does not provide for views of a subject taken along planes parallel to the path of the X-rays through the subject.
In order to overcome these problems various tomographic techniques have heretofore been developed for obscuring data in the image that derives from regions of the subject located apart from a selected plane. One simple technique for accomplishing this result in conventional radiology is to move the X-ray source and the photographic film in opposite but parallel directions during exposure. Under this condition only one plane within the subject remains in sharp focus on the film. While this can be useful in many situations, one's ability to distinguish between data in the plane of interest from the blurred background image data is still considerably less than would be desirable. Further, the technique is limited insofar as variation of the orientation of the imaged plane is concerned.
To overcome these limitations of older procedures, much effort has recently been made to develop a new technique known as computed tomography. In most computed tomographic X-ray systems as heretofore constructed, a conventional point X-ray tube is situated on one side of the subject and a point X-ray detector on the other. The source and detector are then moved synchronously in parallel directions to perform a rectilinear scan across a plane transecting the subject. The output of the detector in the course of this scan is representative of variations of X-ray transmissiveness along a series of parallel zones through the subject which lie in the plane of the scan. This information is stored together with data indicative of the point in the scan at which each unit of such information was generated. The source and detector are then rotated relative to the subject or the subject is rotated and a similar scan is then made at the changed angular position. Typically, the rotational repositioning is repeated a very large number of times with the rectilinear scan being repeated at each angular position. The stored X-ray count information and positional data are then utilized to generate a visible image of a cross-sectional plane through the subject in which areas of different X-ray transmissiveness are clearly defined and readily distinguishable. A variety of techniques for generating the desired cross-sectional planar X-ray image from the stored data are known. These are primarily variations of a technique known as the summation or back projection method which is described for example in the journal RADIOLOGY 87; 278 (1966) in an article by Kuhl and Edwards. Although analog systems have been devised, most of the recently developed tomographic systems of this general kind utilize a digital computer to perform much of the operations required for image generation from the X-ray detector output information and scan position data, such a system also being described in the journal RADIOLOGY 91; 975 (1968) by the same authors.
These improvements in X-ray cross-sectional imaging by computed tomography have proved to be extremely useful in medical radiology, particularly for distinguishing between biological tissues of only slightly varying densities such as occur in the brain, abdomen and chest cavity. The recent development of a computed tomographic brain scanner in particular had led to extreme medical interest in the further possible capabilities of X-ray tomography as a powerful diagnostic tool.
The computed tomographic systems presently known to the art tend to be extremely complex and costly. Size, space and power requirements further limit the availability of such systems to a few large, well financed medical centers. Much of the cost, complexity and size of this apparatus derives from the need to provide precision mechanisms to effect the controlled repetitive rectilinear scanning motion alternated with precise angular repositioning motions of the source and detector structure relative to the subject to be analyzed. Further, because the scanning is accomplished mechanically, undesirably long periods are required to produce a tomographic image. If sectional X-ray images are to be produced for more than one plane through the subject, or for oblique planes, the mechanical positioning and scanning structure becomes even more complex.